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首先给出图1所示的非零位拼接检测凸非球面金属镜的流程图,图2所示的是凸非球面金属镜检测的光路图。
图 1 非零位拼接检测凸非球面金属镜流程图
Figure 1. Flow chart of non-null stitching test of convex aspheric metal mirror
图 2 凸非球面金属反射镜检测原理示意图。(a) 非球面检测示意图;(b) 标准镜选择示意图
Figure 2. Schematic diagram of test principle of convex aspheric metal mirror. (a) Schematic diagram of aspheric surface test; (b) Schematic diagram of standard mirror selection
从图1中可得,在检测凸非球面金属镜时首先需要根据被检镜参数选择标准镜,并根据被检镜参数和选择的标准镜参数,进行子孔径规划。
由图2可得,凸非球面金属镜检测时球面的标准镜选择原则:
(1) F/#≥R/#(F/#= f/D, R/#=R/d,其中F/#为标准镜F数,R/#为被检镜R数,f为标准镜焦距,D为标准镜口径,R为被检镜顶点曲率半径,d为被检镜口径)。
(2) f >R。
子孔径规划原则:
(1)各子孔径干涉条纹数小于干涉仪最大分辨条纹数(实验中使用干涉仪CCD相机分辨率为45对条纹,并且要求干涉检测波前均方根值(RMS)<4λ)。
(2)保证规划的子孔径能够对全口径面形实现覆盖。
(3)各相邻子孔径检的重叠面积≥30%[14]。
在进行拼接检测实验前,首先需要计算被检镜的非球面度,以判断是否能够采用非零位拼接检测方法检测。在分析完成后,根据被检镜参数和选择的标准镜参数合理规划子孔径。为保证检测精度,各子孔径重叠面积应超过30%[14]。文中针对表1所示参数的凸非球面金属反射镜进行了仿真分析和实验验证,以验证非零拼接检测凸非球面金属反射镜的可行性。
表 1 被检镜参数
Table 1. Parameters of the tested mirror
Parameter Value Diameter/mm 120 Vertex radius of curvature/mm 1121.586 Conic constant −2.38 A6 1.83×10−15 A8 7.91×10−19 A10 1.05×10−22 首先根据表1所示参数,分析被检镜非球面度,结果如图3(a)所示,被检镜非球面度峰值(PV)为0.667λ,由图3(b)得,若以最大非球面度检测被检镜全口径面形,其干涉条纹数量为22根条纹。实验中使用的Zygo干涉仪最大分辨为90根条纹,即可以采用非零位拼接检测的方法检测被检镜。
图 3 被检镜非球面度。 (a) 被检镜非球面度计算结果;(b) 全口径Slope 分析结果
Figure 3. Asphericity of the tested mirror. (a) Calculation result of the asphericity of the tested mirror; (b) Full-aperture Slope analysis result
根据表1所示被检参数,选择为6 in (1in=2.54 cm),F/#=11的球面标准镜,子孔径规划如图4所示,共需要5个子孔径,即可实现对被检镜全口径面形的检测。
在子孔径规划完成后,对内外两圈子孔径进行仿真分析,中心和外围子孔径的波前图和干涉条纹图如图5所示。可得中心子孔径的设计残差RMS为0.208λ,外围子孔径的设计残差RMS为0.867λ,其设计残差即为非零位误差,可以将设计残差结果作为系统误差,在数据处理过程中予以剔除,从而获得真实的检测面形误差。
图 5 各子孔径面形和干涉条纹仿真结果。(a) 中心子孔径波前;(b) 中心子孔径干涉条纹;(c) 第二圈子孔径波前;(d) 第二圈子孔径干涉条纹
Figure 5. Surface shape of various sub-aperture and interference fringe simulation results. (a) Central sub-aperture wavefront; (b) Central sub-aperture interference fringe; (c) Second circle aperture wavefront; (d) Second circle aperture interference fringe
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图6为凸非球面金属反射镜的拼接检测光路,干涉仪和载物台共同组成了五维调节机构,能够满足凸非球面金属反射镜拼接检测时需要的调整量。同时,金属镜面形易受温度影响,因此需要控制检测时的温度(温度控制在(22±0.2)℃),以保证检测面形的一致性。各子孔径检测结果如图7(a)~(e)所示。
图 7 拼接检测结果。(a)中心子孔径检测结果;(b)~(e)外围子孔径检测结果;(f)拼接后全口径面形
Figure 7. Stitching test results. (a) Center sub-aperture test results; (b)-(e) Peripheral sub-aperture test results; (f) Full-aperture surface shape after stitching
对数据进行处理,以图3子孔径规划的中心子孔径坐标系为基准坐标系[15-16],将各子孔径坐标统一到基准坐标系下,如公式(1)所示:
$$\varPhi _i'(x,y) = {\varPhi _i}(x,y) + \sum\limits_{k = 1}^L {{a_{ik}}{f_k}(x,y)} $$ (1) 式中:Φi(x,y)为第i个子孔径的检测结果;Φ'i(x,y)为第i个子孔径在基准坐标系下的坐标;aik为拼接系数;k为拼接系数所对应方程的序号,由于检测面形为凸非球面元件所以L=9,fk设置如公式(2)所示的九个表达式:
$${f_k} = \left\{ {\begin{array}{*{20}{c}} {{f_1}(x,y) = x} \\ {{f_2}(x,y) = y} \\ {{f_3}(x,y) = {x^2} + {y^2}} \\ {{f_4}(x,y) = xy} \\ {{f_5}(x,y) = {x^2} - {y^2}} \\ {{f_6}(x,y) = x({x^2} + {y^2})} \\ {{f_7}(x,y) = y({x^2} + {y^2})} \\ {{f_8}(x,y) = {{({x^2} + {y^2})}^2}} \\ {{f_9} = 1} \end{array}} \right.$$ (2) $$\delta^{2}=\displaystyle\sum\limits_{i=1 \ldots n} \sum\limits_{j=1 \ldots n \\ j \ne i}^{i \cap j}\left[\begin{array}{c} \left(\varPhi_{i}(x, y)+\displaystyle\sum\limits_{k=1}^{L} a_{i k} f_{k}(x, y)\right) \\ -\left(\varPhi_{j}(x, y)+\displaystyle\sum\limits_{k=1}^{L} a_{j k} f_{k}(x, y)\right) \end{array}\right]^{2}=\min$$ (3) 求解拼接系数,使用最小二乘法对公式(3)求解,可得各子孔径对应的的拼接系数aik,从而实现全口径面形的拼接获得以基准坐标系为参考的非球面表面误差分布。其拼接结果如图6(f)所示,拼接后全口径面形RMS为0.250λ。
对拼接结果进行分析,去除检测结果中的非零位误差。去除非零位误差后,被检镜全口径面形误差如图8(a)所示,RMS为0.016λ。为验证该方法的准确性,利用纳米轮廓仪Luphoscan对该金属凸非球面反射镜进行检测,其面形检测结果如图8(b)所示。将两种检测结果对比,其绝对偏差(数值相减)为PV=0.061λ,RMS=0.004λ,拼接检测结果相对于Luphoscan检测结果的残差(数据点对点相减)如图8所示,PV=0.052λ,RMS=0.007λ。实验结果表明非零位拼接检测能够满足凸非球面金属反射镜表面面形的高精度检测,验证了非零位拼接检测法检测凸非球面金属反射镜的可行性。
Non-null stitching test convex aspheric metal mirror
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摘要: 随着单点金刚石车削技术和抛光技术的发展,实现了金属反射镜的快速高效低成本制造。然而,金属反射镜的检测手段存在明显不足,尤其是没有一种快速、高效的检测手段用于检测凸非球面金属反射镜。为提高凸非球面金属反射镜的检测效率,提出一种非零位拼接检测凸非球面金属反射镜的检测方法。结合工程实例,对口径为120 mm,顶点曲率半径R为1121.586 mm,二次曲线常数K为−2.38的凸非球面金属反射镜进行了拼接检测实验,拼接所得面形误差均方根值(RMS)为0.016λ(λ=632.8 nm)。与Luphoscan检测结果对比,验证了非零位拼接检测方法的检测精度RMS为0.007λ,结果表明该方法能够实现凸非球面金属反射镜的快速、高效检测。Abstract: With the development of single-point diamond turning technology (SPDT) and polishing technology, the rapid, efficient and low-cost manufacturing of metal mirrors has been realized. However, the test methods of metal mirrors have obvious shortcomings, especially there is no fast and efficient test method for testing convex aspheric metal mirrors. In order to improve the test efficiency of convex aspheric metal mirrors, a non-null stitching method to test convex aspheric metal mirrors was proposed. Combined with engineering examples, a stitching test experiment was carried out on a convex aspheric metal mirror with a diameter of 120 mm, a radius of curvature of the vertex R of 1121.586 mm, and a conic constant K of −2.38. The residual surface shape obtained by stitching RMS=0.016λ(λ=632.8 nm). Compared with the Luphoscan test results, it is verified that the test accuracy of the non-null stitching test method RMS=0.007λ, which shows this test method can achieve rapid and efficient test of convex aspheric metal mirrors.
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Key words:
- subaperture stitching /
- convex aspheric surface /
- non-null test /
- metal mirror
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图 5 各子孔径面形和干涉条纹仿真结果。(a) 中心子孔径波前;(b) 中心子孔径干涉条纹;(c) 第二圈子孔径波前;(d) 第二圈子孔径干涉条纹
Figure 5. Surface shape of various sub-aperture and interference fringe simulation results. (a) Central sub-aperture wavefront; (b) Central sub-aperture interference fringe; (c) Second circle aperture wavefront; (d) Second circle aperture interference fringe
表 1 被检镜参数
Table 1. Parameters of the tested mirror
Parameter Value Diameter/mm 120 Vertex radius of curvature/mm 1121.586 Conic constant −2.38 A6 1.83×10−15 A8 7.91×10−19 A10 1.05×10−22 -
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